Adapter lens system for use with photographic cameras

ABSTRACT

A triplet type of adapter lens system for use with photographic cameras comprising a first positive lens element, a second biconcave lens element, a third positive lens element and a stop arranged on the front side of said first lens element, said adapter lens system having a short total length, a long back focal length and favorable corrected aberrations.

BACKGROUND OF THE INVENTION

(a) Field of the Invention:

The present invention relates to an adapter lens system for use withphotographic cameras which are utilized for photographing images formedby optical instruments for observation directly by man's eyes such asendoscopes, microscopes and telescopes.

(b) Discription of the prior art:

FIG. 1 shows a sectional view illustrating composition of an opticalsystem of a telescope consisting of an objective lens 1 and an eyespiece2. As is seen from this drawing, the optical system is so designed as topermit man's eye to observe an image formed at a nearly infinitedistance by the eyepiece 2.

As a method for photographing an image formed by such a telescope, therehas conventionally been known a means to shift the eyepiece 2 away fromthe objective lens 1 as shown in FIG. 2 (in the direction indicated byan arrow) so that a real image formed by the objective lens 1 isrefocused on a film 4 by the eyepiece. However, this method makescorrection of aberrations rather difficult since it uses the eyepiece 2at different imaging magnifications levels for observation by man's eyeand photographing an image. Speaking concretely, aberrations areaggravated in the optical system when it is set either for observationby man's eye or photographing with a camera.

As a means for correcting this defect, there has been known anothermethod to employ an adapter lens as shown in FIG. 3. Speakingconcretely, this method use a technique to arrange an adapter lens 3 onthe rear side of the position of the eyepiece 2 for observation by man'seye so that the image formed at nearly infinite distance by the eyepiece2 is refocused on the film 4. This method mades it possibly to permitphotographing with favorably corrected aberrations so long asaberrations of the eyepiece 2 are corrected favorably for observation byman's eye.

A lens system used for this purpose (adapter lens system) may have alarge F number for magnifying an image at a high ratio and a relativelysimple composition. For adding an adapter lens system on the rear sideof an eyepiece of an ocular observation instrument, however, a stop 5 ofthe adapter lens system 3 becomes the exit pupil of the eyepiece 2 andis located on the front side of the adapter lens system 3. When an imageis photographed by using a body of a commercially availablysingle-reflex lens type of camera, on the other hand, it is necessary toprolong the back focal length of the adapter lens system 3 to a certaindegree as is understood from FIG. 4.

The back focal length must be prolonged also when a half prism (a prismcomprising a semi-transparent surface) for automatic exposure control isarranged on the rear side of the adapter lens system 3 as shown in FIG.5. However, a large total length of the adapter lens system or a camerabody will be inconvenient for photographing operations. Therefore, it isdesired for the adapter lens system to have a total length as short aspossible.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide a photographingadapter lens system having a short total length and favorably correctedaberrations.

The adapter lens system according to the present invention is a triplettype comprising a stop arranged on the front side of its lens elementsas shown in FIG. 6. Speaking concretely, the adapter lens systemcomprises a stop, a first positive lens element, a second lens elementand a third positive lens element, said second lens element beingdesigned as a biconcave element for correcting spherical aberration,coma and so on.

Further, the adapter lens system according to the present invention isso designed as to satisfy conditions (1) to (6) specified below:

(1) f_(B) >0.85f

(2) L<0.35f

(3) f₁ >0.5f

(4) 0.06f≦d₃

(5) ν₁ <45

(6) d₅ ≧0.05f

wherein the reference symbols are defined as follows:

L: total length of said adapter lens system as a whole

f_(B) : distance as measured from the image side surface of said thirdlens element to the rear focal point of the lens system (back focallength)

f₁ : focal length of said first lens element

f: focal length of the adapter lens system as a whole

d₃ : airspace reserved between said first and second lens elements

d₅ : airspace reserved between said second and third lens elements

ν₁ : Abbe's number of said first lens element

Now, significance of the individual conditions will be described indetails:

The condition (1) defines a back focal length so as to enable attachingthe adapter lens system to a single-reflex lens camera and arranging alight measuring prism on the rear side of said adapter lens system. Ifthe backfocal length of the adapter lens system does not satisfy thiscondition, it will be impossible to attach the adapter lens system to asingle-reflex lens camera or arranged a light measuring prism in saidadapter lens system.

The condition (2) specifies a total length of said adapter lens system.If the total length of the adapter lens system exceeds the limit definedby the condition (2), photographing operations will be ratherinconvenient.

The condition (3) defines power of the first lens element. In order toobtain a long back focal length, it is necessary to shift the principalpoint of the lens system rearward. For this purpose, the first lenselement must have a low power. If the condition (3) is not satisfied, itwill be impossible to prolong the back focal length of the adapter lenssystem.

The condition (4) defines airspace to be reserved between the first andsecond lens elements. The condition (2) has been adopted since theadapter lens system of the concerned type must have a short totallength. In this respect, it is desirable that d₃ be as short aspossible. When d₃ is too narrow, however, it will be impossible tocorrect aberrations sufficiently favorably. In case of a triplet type oflens system comprising a stop on the front side of the first lenselement, coma due to the upper rays can be positively correctedsubstantially by the rear surface only of the concave lens element (theimage side surface of the second lens element). When d₃ is narrow, theupper rays height is low on the rear side surface of the second lenselement, thereby weakening the function to positively correcting coma.When the said lens surface is so designed as to have a small radius ofcurvature to enhance this function, the front side surface of the secondlens element will have a large radius of curvature, thereby resulting inundercorrection of spherical aberration.

As is uncerstood from the foregoing description, the condition (4) isnecessary for preventing spherical aberration from being undercorrectedwhen coma is corrected favorably. Therefore, spherical aberration willbe undercorrected if the condition (4) is not satisfied.

The condition (5) has been adopted for preventing longitudinal chromaticaberration from being overcorrected when lateral chromatic aberration iscorrected favorably.

The adapter lens system according to the present invention is sodesigned as to correct lateral chromatic aberration favorably byutilizing the diverging function of the second lens element so as tocancel the positive functions of the other lens elements. When thedispersing power of the second lens element is so defined as to correctlateral chromatic aberration favorably, correcting function forlongitudinal chromatic aberration will be too strong and result inovercorrection of longitudinal chromatic aberration. In order to preventsuch overcorrection of longitudinal chromatic aberration, it isnecessary to cancel the longitudinal chromatic aberration produced bythe second lens element by imparting high dispersing power to the firstlens element on which the paraxial rays incident at its higher section.Overcorrection of longitudinal chromatic aberration has been preventedby this concept. Longitudinal chromatic aberration will be overcorrectedif the condition (5) is not satisfied.

The condition (6) has been adopted to prevent the airspace reservedbetween the second and third lens elements from being too short. Whenthe second lens element is located too close to the third lens elementby designing the adapter lens system so as to satisfy the conditions (1)and (2), both the second and third lens elements will have too shortfocal lengths, thereby remarkably curving coma. Though a short airspaced₅ between the second and third lens elements is desirable forshortening the total length of the entire lens system, d₅ shorter thanthe limit defined by the condition (6) will aggravate coma for thereason described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view illustrating composition of an opticalsystem of a telescope;

FIG. 2 and FIG. 3 show sectional views illustrating the methods forphotographing an image formed by said telescope;

FIG. 4 shows a sectional view illustrating the positional relationshipbetween the lens elements and a stop for the method shown in FIG. 3;

FIG. 5 shows a sectional view in which an adapter lens systemm isattached to a photographing camera;

FIG. 6 shows a sectional view illustrating the composition of theadapter lens system according to the present invention; and

FIG. 7 through FIG. 11 respectively show curves illustrating theaberration characteristics of Embodiments 1 through 5 of the adapterlens system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, some preferred embodiments of the adapter lens system according tothe present invention will be described below:

    ______________________________________                                        Embodiment 1                                                                  ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4128                                                              r.sub.2 = 40.0974                                                             d.sub.2 = 5.0477                                                                             n.sub.1 = 1.60342                                                                           ν.sub.1 = 38.01                               r.sub.3 = ∞                                                             d.sub.3 = 7.2199                                                              r.sub.4 = -52.2462                                                            d.sub.4 = 2.3556                                                                             n.sub.2 = 1.68893                                                                           ν.sub.2 = 31.08                               r.sub.5 = 40.1509                                                             d.sub.5 = 8.9327                                                              r.sub.6 = 111.5114                                                            d.sub.6 = 5.0477                                                                             n.sub.3 = 1.62041                                                                           ν.sub.3 = 60.27                               r.sub.7 = -38.5050                                                            d.sub.7 = 2.5239                                                              r.sub.8 = ∞                                                             d.sub.8 = 23.5559                                                                            n.sub.4 = 1.51633                                                                           ν.sub.4 = 64.15                               r.sub.9 = ∞                                                             f = 100,                                                                      object point 841.284, image height 13.461                                     f.sub.B = 94.864                                                                            L = 28.61  f.sub.1 =  66.45                                     ______________________________________                                        Embodiment 2                                                                  ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 10.0365                                                             r.sub.2 = 44.8883                                                             d.sub.2 = 3.3680                                                                             n.sub.1 = 1.60717                                                                           ν.sub.1 = 40.26                               r.sub.3 = ∞                                                             d.sub.3 = 10.306                                                              r.sub.4 = -38.2298                                                            d.sub.4 = 3.7721                                                                             n.sub.2 = 1.64769                                                                           ν.sub.2 = 33.80                               r.sub.5 = 38.2298                                                             d.sub.5 = 6.1292                                                              r.sub.6 = 80.4927                                                             d.sub.6 = 5.0520                                                                             n.sub.3 = 1.58913                                                                           ν.sub.3 = 60.97                               r.sub.7 = -31.1672                                                            d.sub.7 = 2.5260                                                              r.sub.8 = ∞                                                             d.sub.8 = 23.5758                                                                            n.sub.4 = 1.51633                                                                           ν.sub.4 = 64.15                               r.sub.9 = ∞                                                             f = 100,                                                                      object point 841.992, image height 13.472                                     f.sub.B = 94.891,                                                                           L = 28.62, f.sub.1 = 73.93                                      ______________________________________                                        Embodiment 3                                                                  ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4110                                                              r.sub.2 = 47.4856                                                             d.sub.2 = 3.3644                                                                             n.sub.1 = 1.64769                                                                           ν.sub.1 = 33.80                               r.sub.3 = ∞                                                             d.sub.3 = 9.1478                                                              r.sub.4 = -48.9170                                                            d.sub.4 = 4.3722                                                                             n.sub.2 = 1.72825                                                                           ν.sub.2 = 28.46                               r.sub.5 = 48.9170                                                             d.sub.5 = 7.5075                                                              r.sub.6 = 131.8984                                                            d.sub.6 = 4.2055                                                                             n.sub.3 = 1.69680                                                                           ν.sub.3 = 55.52                               r.sub.7 = -39.7164                                                            d.sub.7 = 2.5233                                                              r.sub.8 = ∞                                                             d.sub.8 = 23.5508                                                                            n.sub.4 = 1.51633                                                                           ν.sub.4 = 64.15                               r.sub.9 = ∞                                                             f = 100,                                                                      object point 841.0995, image height 13.458                                    f.sub.B = 94.815,                                                                           L = 31.12, f.sub.1 = 73.32                                      ______________________________________                                        Embodiment 4                                                                  ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4189                                                              r.sub.2 = 40.2970                                                             d.sub.2 = 3.3676                                                                             n.sub.1 = 1.61293                                                                           ν.sub.1 = 37.00                               r.sub.3 = ∞                                                             d.sub.3 = 8.3588                                                              r.sub.4 = -49.8022                                                            d.sub.4 = 3.4457                                                                             n.sub.2 = 1.68893                                                                           ν.sub.2 = 31.08                               r.sub.5 = 37.8875                                                             d.sub.5 = 8.4009                                                              r.sub.6 = 92.2409                                                             d.sub.6 = 5.0514                                                                             n.sub.3 = 1.58913                                                                           ν.sub.3 = 60.97                               r.sub.7 = -35.7511                                                            d.sub.7 = 2.5257                                                              r.sub.8 = ∞                                                             d.sub.8 = 23.5730                                                                            n.sub.4 = 1.51633                                                                           ν.sub.4 = 64.15                               r.sub.9 = ∞                                                             f = 100,                                                                      object point 841.89, image height 13.470                                      f.sub.B = 94.914,                                                                           L = 28.62, f.sub.1 = 65.75                                      ______________________________________                                         Embodiment 5                                                                 ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4222                                                              r.sub.2 = 35.9119                                                             d.sub.2 = 5.0533                                                                             n.sub.1 = 1.58144                                                                           ν.sub.1 = 40.75                               r.sub.3 = 189.9936                                                            d.sub.3 = 6.7377                                                              r.sub.4 = -60.6219                                                            d.sub.4 = 3.3689                                                                             n.sub.2 = 1.66680                                                                           ν.sub.2 = 33.04                               r.sub.5 = 34.5753                                                             d.sub.5 = 7.58                                                                r.sub.6 = 71.9651                                                             d.sub.6 = 6.7377                                                                             n.sub.3 = 1.62041                                                                           ν.sub.3 = 60.27                               r.sub.7 = -41.5271                                                            d.sub.7 = 2.5267                                                              r.sub.8 = ∞                                                             d.sub.8 = 23.5821                                                                            n.sub.4 = 1.51633                                                                           ν.sub.4 = 64.15                               r.sub.9 = ∞                                                             f = 100,                                                                      object point 842.219, image height 13.476                                     f.sub.B = 96.455,                                                                           L = 29.48, f.sub.1 = 75.25                                      ______________________________________                                    

wherein the reference symbols r₁ through r₉ represent radii of curvatureon the surfaces of the respective lens elements, the reference symbolsd₁ through d₈ designate thicknesses of the respective lens elements andairspaces reserved between the respective lens elements, the referencesymbols n₁ through n₄ denote refractive indices of the respective lenselements, and the reference symbols ν₁ through ν₄ represent Abbe'snumbers of the respective lens elements.

The numerical data of the embodiments described above contain those ofstops and light measuring prisms. That is to say, the reference symbolr₁ represents radius of curvature of a stop, the reference symbol d₁designates a distance between a stop and the object side surface of thefirst lens element, the reference symbols r₈ and r₉ denote radii ofcurvature on both the surfaces of prism P, the reference symbol d₈represents thickness thereof, the reference symbol n₄ designatesrefractive index of prism P and the reference symbol ν₄ denotes Abbe'snumber thereof.

Aberrations of these embodiments are illustrated in FIG. 7 through FIG.11 as those including aberrations produced by the prsim P.

As is clearly understood from the foregoing descriptions andembodiments, the adapter lens system according to the present inventionis a triplet type comprising a stop arranged on the front side of thelens elements and the second lens element designed as a biconcaveelement. The lens system is short in its total length, favorablycorrected in aberrations and desirable as an adapter lens system for usewith photographic cameras since it is so designed as to safisfy theconditions (1) through (6) described above.

Each embodiment is usable as a photographic camera lens since it hasabout +1.19 diopters and an object point located nearly at an infinitedistance.

I claim:
 1. An adapter lens system for use with photographic camerascomprising a first positive lens element, a second biconcave lenselement, and a third positive lens element and a stop arranged on thefront side of said lens elements, said lens system satisfying thefollowing condition:(1) f_(B) >0.85f (2) L<0.35f (3) f₁ >0.5f (4)0.06f≦d₃ (5) ν₁ <45 (6) d₅ ≧0.05fwherein the reference symbol Lrepresents total length of said lens system as a whole, the referencesymbol f_(B) designates back focal length of said lens system, thereference symbol f₁ denotes focal length of said first lens element, thereference symbol f represents focal length of said lens system as awhole, the reference symbol d₃ designates airspace reserved between saidfirst and second lens elements, the reference symbol d₅ denotes airspacereserved between said second and third lens elements and the referencesymbol ν₁ represents Abbe's number of said first lens element.
 2. Anadapter lens system for use with photographic cameras according to claim1 having the following numerical data:

    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4128                                                              r.sub.2 = 40.0974                                                             d.sub.2 = 5.0477                                                                             n.sub.1 = 1.60342                                                                           ν.sub.1 = 38.01                               r.sub.3 = ∞                                                             d.sub.3 = 7.2199                                                              r.sub.4 = -52.2462                                                            d.sub.4 = 2.3556                                                                             n.sub.2 = 1.68893                                                                           ν.sub.2 =31.08                                r.sub.5 = 40.1509                                                             d.sub.5 = 8.9327                                                              r.sub.6 = 111.5114                                                            d.sub.6 = 5.0477                                                                             n.sub.3 = 1.62041                                                                           ν.sub.3 = 60.27                               r.sub.7 = -38.5050                                                            f = 100,                                                                      object point 841.284, image height 13.461                                     f.sub.B = 94.864,                                                                           L = 28.61, f.sub.1 = 66.45                                      ______________________________________                                    

wherein the reference symbols r₁ through r₇ represent radii of curvatureon the surfaces of the respective lens elements, the reference symbolsd₁ through d₃ designate the thicknesses of the respective lens elementsand airspaces reserved between the respective lens elements, thereference symbols n₁ through n₃ denote refractive indices of therespective lens elements and the reference symbols ν₁ through ν₃represent Abbe's numbers of the respective lens elements.
 3. An adapterlens system for use with photographic cameras according to claim 1having the following numerical data:

    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 10.0365                                                             r.sub.2 = 44.8883                                                             d.sub.2 = 3.3680                                                                             n.sub.1 = 1.60717                                                                           ν.sub.1 = 40.26                               r.sub.3 = ∞                                                             d.sub.3 = 10.306                                                              r.sub.4 = -38.2298                                                            d.sub.4 = 3.7721                                                                             n.sub.2 = 1.64769                                                                           ν.sub.2 = 33.80                               r.sub.5 = 38.2298                                                             d.sub.5 = 6.1292                                                              r.sub.6 = 80.4927                                                             d.sub.6 = 5.0520                                                                             n.sub.3 = 1.58913                                                                           ν.sub.3 = 60.97                               r.sub.7 = -31.1672                                                            f = 100                                                                       object point 841.992, image height 13.472                                     f.sub.B = 94.891,                                                                           L = 28.62, f.sub.1 = 73.93                                      ______________________________________                                    

wherein the reference symbols r₁ through r₇ represent radii of curvatureon the surfaces of the respective lens elements, the reference symbolsd₁ through d₆ designate the thicknesses of the respective lens elementsand airspaces reserved between the respective lens elements, thereference symbols n₁ through n₃ denote refractive indices of therespective lens elements and the reference symbols ν₁ through ν₃represent Abbe's numbers of the respective lens elements.
 4. An adapterlens system for use with photographic cameras according to claim 1having the following numerical data:

    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4110                                                              r.sub.2 = 47.4856                                                             d.sub.2 = 3.3644                                                                             n.sub.1 = 1.64769                                                                           ν.sub.1 = 33.80                               r.sub.3 = ∞                                                             d.sub.3 = 9.1478                                                              r.sub.4 = -48.9170                                                            d.sub.4 = 4.3722                                                                             n.sub.2 = 1.72825                                                                           ν.sub.2 = 28.46                               r.sub.5 = 48.9170                                                             d.sub.5 = 7.5075                                                              r.sub.6 = 131.8984                                                            d.sub.6 = 4.2055                                                                             n.sub.3 = 1.69680                                                                           ν.sub.3 = 55.52                               r.sub.7 = -39.7164                                                            f = 100,                                                                      object point 841.0995, image height 13.458                                    f.sub.B = 94.815,                                                                           L = 31.12, f.sub.1 = 73.32                                      ______________________________________                                    

wherein the reference symbols r₁ through r₇ represent radii of curvatureon the surfaces of the respective lens elements, the reference symbolsd₁ through d₆ designate the thicknesses of the respective lens elementsand airspaces reserved between the respective lens elements, thereference symbols n₁ through n₃ denote refractive indices of therespective lens elements and the reference symbols ν₁ through ν₃represent Abbe's numbers of the respective lens elements.
 5. An adapterlens system for use with photographic cameras according to claim 1having the following numerical data:

    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4189                                                              r.sub.2 = 40.2970                                                             d.sub.2 = 3.3676                                                                             n.sub.1 = 1.61293                                                                           ν.sub.1 = 37.00                               r.sub.3 = ∞                                                             d.sub.3 = 8.3588                                                              r.sub.4 = -49.8022                                                            d.sub.4 = 3.4457                                                                             n.sub.2 = 1.68893                                                                           ν.sub.2 = 31.08                               r.sub.5 = 37.8875                                                             d.sub.5 = 8.4009                                                              r.sub.6 = 92.2409                                                             d.sub.6 = 5.0514                                                                             n.sub.3 = 1.58913                                                                           ν.sub.3 = 60.97                               r.sub.7 = -35.7511                                                            f = 100,                                                                      object point 841.89, image height 13.470                                      f.sub.B = 94.914,                                                                           L = 28.62, f.sub.1 = 65.75                                      ______________________________________                                    

wherein the reference symbols r₁ through r₇ represent radii of curvatureon the surfaces of the respective lens elements, the reference symbolsd₁ through d₆ designate the thicknesses of the respective lens elementsand airspaces reserved between the respective lens elements, thereference symbols n₁ through n₃ denote refractive indices of therespective lens elements and the reference symbols ν₁ through ν₃represent Abbe's numbers of the respective lens elements.
 6. An adapterlens system for use with photograhic cameras according to claim 1 havingthe following numerical data:

    ______________________________________                                        r.sub.1 = ∞                                                             d.sub.1 = 8.4222                                                              r.sub.2 = 35.9119                                                             d.sub.2 = 5.0533                                                                             n.sub.1 = 1.58144                                                                           ν.sub.1 = 40.75                               r.sub.3 = 189.9936                                                            d.sub.3 = 6.7377                                                              r.sub.4 = -60.6219                                                            d.sub.4 = 3.3689                                                                             n.sub.2 = 1.66680                                                                           ν.sub.2 = 33.04                               r.sub.5 = 34.5753                                                             d.sub.5 = 7.58                                                                r.sub.6 = 71.9651                                                             d.sub.6 = 6.7377                                                                             n.sub.3 = 1.62041                                                                           ν.sub.3 = 60.27                               r.sub.7 = -41.5271                                                            f = 100,                                                                      object point 842.219, image height 13.476                                     f.sub.B = 96.455,                                                                           L = 29.48, f.sub.1 = 75.25                                      ______________________________________                                    

wherein the reference symbols r₁ through t₇ represent radii of curvatureon the surfaces of the respective lens elements, the reference symbolsd₁ through d₆ designate the thicknesses of the respective lens elementsand airspaces reserved between the respective lens elements, thereference symbols n₁ through n₃ denote refractive indices of therespective lens elements and the reference symbols ν₁ through ν₃represent Abbe's numbers of the posiective lens elements.